Numerical phase-detection autofocusing method for digital holography reconstruction processing

Abstract

Digital holography requires precise phase analysis without wavefront aberration from defocusing errors. While the auto-contrast method is generally advantageous for refocusing, its accuracy suffers in low contrast conditions. Here, we suggest a simple phase analysis approach applying phase-detection autofocusing (PDAF) using a Fourier operation and the angular spectrum method to achieve fast and accurate refocusing even with high transmittance samples. The speed and sub-nano accuracy of the proposed method along with its quantitative autofocusing capability make it well suited for numerical digital holography reconstruction. Phase analysis with the PDAF technique has sensitivity to 0.1 nm changes in the focusing distance within the light source wavelength. In the reconstruction process, this method causes no wavefront distortion. The refocus calculation time using the focus calibration function obtained through the PDAF method is 0.012 s, enabling a real-time refocus correction of 83 fps in digital video holography. We performed refractive index measurements of Boro33 plates with high transmittance to interpret the physical meaning of the corrected focal distances and obtained a refractive index accuracy of 0.001. We then applied a real-time digital holographic measurement system and found that the PDAF technique removed dynamic phase fluctuation, an issue that cannot be solved by phase unwrapping.